A month before he shot and killed Oscar Grant, former BART police Officer Johannes Mehserle was given Taser training that stressed the importance of not confusing the weapon with a gun - and was even told of cases in which police had reported shooting people by making such a mistake.

That was the testimony Wednesday of Stewart Lehman, a BART police officer who trained Mehserle on Dec. 3, 2008, to use Tasers. He was called as a witness for Mehserle, who is charged with murdering the unarmed Grant while trying to arrest and handcuff him on Jan. 1, 2009, at BART's Fruitvale Station in Oakland.

Lehman's testimony deepened the central debate in the case, with defense attorney Michael Rains trying to use him to show that Mehserle had been afflicted by the "Taser confusion" discussed during his training.

Prosecutor David Stein, meanwhile, sought to strengthen his argument that the Taser scenario makes no sense, given the circumstances on the station platform that led to Grant's shooting and the differences between a Taser and a firearm.

In his questioning of Lehman, Rains suggested that the Taser training given to Mehserle and other BART officers had been inadequate because officers were allowed to fire the device just once over the course of six hours. Lehman said the training met standards set by a state oversight agency.

Once they were armed with the Tasers, officers were allowed to choose where to carry them. But, Lehman said, a right-handed officer such as Mehserle was not allowed to position the Taser next to the pistol on the right hip unless he aimed the weapon's handle in the opposite direction, making only a left-handed draw possible.

"The intent of that was to avoid the weapons confusion issue," Lehman said.

Lehman also testified that officers in Madera, Sacramento and Rochester, N.Y., had reported shooting people by mixing up a Taser and a gun.

Stein, in his cross-examination, had Lehman demonstrate the difference between pulling a Taser and a gun from a holster. He gave Lehman the duty belt that Mehserle had used, then had him stand up and draw Mehserle's yellow Taser X26.

The night he killed Grant, Mehserle wore his Taser to the left of his belt buckle, with the holster configured for a right-handed draw.

To pull the Taser in the courtroom demonstration, Lehman had to unsnap a button with his right thumb. Then, with the same thumb, he rolled forward a hood over the Taser before pulling the weapon.

Earlier, Stein showed jurors how Mehserle would have had to pull his black Sig Sauer P226 pistol. The gun holster requires an officer to roll forward a hood with the thumb of his gun hand, then use the same thumb to push back a lever that frees up the firearm.

Stein suggested that the three incidents in which officers reported mistaking a gun for a Taser were different in important ways from Mehserle's case. Those officers carried an earlier Taser model, the M26, and all were carrying the Tasers on their gun side - the opposite of Mehserle.

Rains and Stein also sparred over the significance of Mehserle getting up off of Grant before shooting him, an action caught on video.

Lehman said officers were trained that if they were at least a few feet from a subject, the two electric probes fired by a Taser would be able to spread out. The device sends electricity between those probes, locking up muscles.

Under Stein's cross-examination, Lehman said the officers also were trained that they could fire the probes into a subject from a short distance, then press the Taser itself into the person to "achieve the spread."